This application claims priority of German Application No. 101 23 097.4, filed May 7, 2001, the complete disclosure of which is hereby incorporated by reference.
a) Field of the Invention
The invention is directed to a tool head such as can be used for all material-removing or cutting processes by means of laser and as is known generically from DE 295 04 457 U1.
b) Description of the Related Art
In material-removing or cutting processes by means of laser, reaction materials (particles) are released in the cutting zone which move away from the surface of the workpiece randomly in an explosive manner. When these reaction materials come to rest and are deposited in the area of the beam path of the laser beam bundle (optical channel), they can hinder the laser machining process uncontrollably.
Such hindrances can be brought about on the one hand by a change in the transmission conditions in the optical channel and, on the other hand, by a geometric limiting of the optical channel.
In practice, impairment of transmission conditions is brought about when optically active surfaces are contaminated or the air paths in the optical channel fill with particles causing increased absorption and scattering of the laser radiation.
The risk of geometric limitation of the optical channel exists particularly when edges and surfaces which are not actually optically active undergo a geometric expansion in the optical channel due to depositing of particles and thereby become optically active.
As a result of the problems described above, the following mandatory requirements are made for a tool head with a laser cutting nozzle:
particles must be prevented from falling on the final optical surface in the direction of the workpiece surface;
the air path must be kept free from particles in the optical channel between the final optical surface and the surface of the workpiece, i.e., these particles must be removed immediately as they occur.
Different solutions are known from the prior art for overcoming these problems which meet the requirements with varying degrees of expenditure and success.
In order to prevent contamination of the focusing optics required for beam shaping, it is known to arrange a protective glass after the focusing optics in the beam direction. The protective glass is exchanged when its contamination impairs machining. This means a cyclically repeating expenditure of time and materials. In order to keep the protective glass clean for longer periods, it is known to blow away particles released during machining by a lateral gas flow and then to suck out the particles (cross-jet principle). In this way, particles located downstream of the axis of the laser beam bundle can be reliably removed, while particles located upstream are deflected and can be blown from the protective glass. Contamination is accordingly slowed down but not reliably prevented.
In order for a gas flow directed transverse to the laser beam bundle to be applied to the machining surface and then sucked out again, the arrangement requires at least one gas nozzle and a suction device transverse to the laser beam bundle.
Due to its expanded construction transverse to the laser beam bundle, a solution of this kind has a increased space requirement directly above the workpiece surface to be machined and is therefore limited to the machining of flat workpieces.
The solution disclosed in Utility Model DE 295 04 457 U1 promises a reduced space requirement and improved prevention of contamination of the protective glass following the focusing optics. The basic idea in this case is to constantly move the released particles out of the optical channel away from the axis of the laser beam bundle and to prevent reaction materials from the environment from entering the optical channel. For this purpose, a nozzle is arranged following the protective glass. The gas flow supplied via an inlet opening is guided through the coaxial arrangement of the nozzle about the axis of the laser beam bundle in its radiating direction and completely encloses the laser beam bundle and, therefore, the optical channel between the outlet opening of the nozzle and the surface of the workpiece. In order to suck out the gas flow, including the entrained particles, the nozzle is surrounded by an annular exhaust funnel. In an arrangement of this kind, the gas flows essentially in axial direction of the laser beam bundle until impacting on the workpiece surface and is then deflected in radial direction away from the axis.
While the solution described above does prevent contamination of the protective glass and ensures rapid removal of particles from the optical channel, it creates a new problem in some cases. The circumstances under which it only becomes a problem are when the selected diameter of the outlet opening of the nozzle should be as small as possible.
Assuming that the diameter of the outlet opening of the nozzle should be as small as possible in order to generate a high kinetic energy which acts against the particle flight due to a high flow velocity even with small amounts of gas, the diameter of the outlet opening should only have minimally greater dimensions than the diameter of the optical channel in this plane. However, this means that even small deposits at the outlet opening can lead to an uncontrolled limiting of the beam bundle and, therefore, to an uncontrolled influence on the laser radiation output impinging on the surface of the workpiece.
As has been shown in practice, the particles accumulate very quickly in radial as well as axial direction to the axis of symmetry of the nozzle at its outlet opening. The outlet opening which is round when in clean condition xe2x80x9cgrowsxe2x80x9d in a funnel-shaped manner in the beam direction as the deposit of particles increases, so that an undefined diaphragm stop effect can be produced on the laser beam bundle.
The same problems can occur in a solution described in DE 39 23 829. In this case, the optics are again followed by a nozzle which surrounds the laser beam bundle coaxially, but which serves to supply a reaction gas. The nozzle is surrounded by an exhaust hood which has a plurality of inlet openings arranged in a ring-shaped manner in the vicinity of the workpiece surface and outlet openings near the nozzle. This construction causes flow ratios under the exhaust hood such that reaction materials are prevented from exiting, which serves essentially to protect work personnel.
DE 197 34 294 describes an exhaust installation for a device for laser machining, particularly for laser welding. In this case, an inner and outer annular nozzle is arranged coaxial to the focusing optics. The inert or protective gas required for welding is supplied through the inner annular nozzle, while the reaction materials and protective gas are sucked out through the outer annular nozzle. No steps are taken to prevent reaction materials from depositing in the optical channel which is enclosed in this case by the inner surface of the inner annular nozzle.
When the prior art described above is considered as a whole, the arrangement of a gas-conducting nozzle surrounding the laser beam bundle (optical channel) shows itself to be very suitable for preventing particles from depositing on the final surface of the focusing optics. However, in order for the nozzle to be dimensioned in an optimal manner for generating a high gas velocity, i.e., with the smallest possible outlet opening, steps must be taken to prevent particles from depositing at its outlet opening in particular.
One possibility consists in keeping the particle flow away from the outlet opening. Since the path of the particle flow is essentially determined by the flow direction of the suction, apart from the impact directions when the particles are detached, contamination of the outlet opening can be extensively prevented when the suction is oriented just above the machining surface radial to the radiating direction of the laser beam bundle as was described in the beginning. An arrangement of this type is not suitable for machining three-dimensional surfaces. Rather, the only arrangements considered are those in which the suction devices are arranged in axial direction around the optical channel so that the freedom of movement of the tool head with respect to three-dimensional surfaces is not limited by the suction device. Therefore, the particle flow also strikes the outlet opening and the adjacent surfaces of the nozzle. The prior art shows no solution in which steps are undertaken for preventing particles from accumulating on the outlet nozzle and the adjacent surface of the nozzle.
It is the primary object of the invention to provide a tool head for laser machining of materials comprising a nozzle which conducts gas and which surrounds the laser beam bundle and comprising a suction device, wherein particles are reliably prevented from depositing on the outlet opening and on the adjacent surface of the nozzle. Further, a design solution is to be found for the tool head that is also suitable for machining workpieces having a three-dimensional surface form.
The object of the invention is met by a tool head for machining of materials by laser comprising a gas-conducting nozzle surrounding the laser beam bundle and a suction device in that the nozzle is coaxially enclosed by a pipe whose inner diameter is greater than the outer diameter of the nozzle in every plane radial to the axis and the first end of the pipe is connected in a gastight manner to the outer shell or surface of the nozzle, in that an inlet opening is provided in the pipe, through which a gas is introduced into the intermediate space formed between the outer surface of the nozzle and the inner surface of the pipe, in that the second end of the pipe is limited to an annular outlet opening by the outer surface of the nozzle, and in that the annular outlet opening is arranged after the outlet opening in the radiating direction of the laser beam bundle, so that the gas flows in axial direction around the exposed outer surface of the nozzle until the outlet opening and subsequently flows to the workpiece surface so as to enclose the free jet exiting from the nozzle.
The invention will be described more fully in the following in two embodiment examples with reference to the drawings.